1. Technical Field
The present invention relates to a pressure sensor element using a pressure sensing element and a diaphragm, and a pressure sensor. In particular, the present invention relates to a technique for reducing errors in a pressure measurement value accompanied by acceleration change.
2. Related Art
Pressure sensors that include a piezoelectric resonator element as a pressure sensing element have been known as water pressure gauges, air gauges, differential pressure gauges, or the like. The piezoelectric resonator element includes, for example, a planar piezoelectric substrate on which an electrode pattern is formed, and a detection axis set in a direction in which force is detected. When pressure is applied in the direction of the detection axis, a resonance frequency of the piezoelectric resonator changes and the pressure is detected from the change of the resonance frequency.
Related art techniques for enhancing accuracy of the pressure sensor are disclosed. JP-A-2007-333452 is a first example of related art. FIG. 13 shows a pressure sensor element disclosed in the first example. As shown in FIG. 13, a pressure sensor element 220 is placed on a pair of supports 218 formed in a diaphragm 212 in a pressure sensor 210. The pressure sensor element 220 includes two bases 222 respectively fixed to the pair of supports 218, and a resonating portion 224 between the bases. Cutouts 222a and 222b are provided between a portion of each base 222 fixed to the support 218 and the resonating portion 224. Accordingly, bending deformation of the resonating portion 224 by the diaphragm 212, i.e., displacement in a thickness direction is concentrated where the cutouts 222a and 222b are formed. Thus, nonlinear force acting on the resonating portion 224 is suppressed, reducing adverse effects on the resonating portion 224 of the pressure sensor element 220.
JP-A-2007-327922 is a second example of related art. FIGS. 14A and 14B show a pressure sensor element disclosed in the second example. FIG. 14A is a schematic diagram when viewed from the front while FIG. 14B is a sectional view taken along the line A-A of FIG. 14A. As shown in FIGS. 14A and 14B, a pressure sensor element 320 is equipped with a piezoelectric resonator element 330 and serves as a pressure sensing element. The pressure sensor element 320 includes a pair of supports 324 for fixing the piezoelectric resonator 330 on a surface of a thin portion 322 serving as a flexible portion, and a projecting portion 326 between the pair of supports 324. The projecting portion 326 is provided so as to increase a thickness of the pressure sensor element 320. Accordingly, deformation of the projecting portion 326 formed between the supports 324 is suppressed, and therefore the projection portion 326 is prevented from being deformed in an arc. Thus, the projecting portion 326 between the supports 324 does not make contact with a resonating portion of the piezoelectric resonator element 330 by the deformation of the projecting portion 326 by a pressure load. Consequently, it is possible to suppress deterioration of detection accuracy of the frequency variation, i.e., deterioration of pressure detection accuracy due to the contact.
JP-A-2008-241287 is a third example of related art. FIG. 15 shows a pressure sensor element disclosed in the third example. As shown in FIG. 15, the pressure sensor element includes a piezoelectric resonator element 431, pedestals 444 and 445, and a thin plate-like diaphragm 440 to which a periphery 442 is fixed. The piezoelectric resonator element 431 includes bases 436 and 437 at both ends of resonating arms 434 and 435. The pedestals 444 and 445 are respectively bonded to the bases 436 and 437. The outline of the diaphragm 440 is formed in substantially rectangular or substantially square. The pedestals 444 and 445 are disposed at the both ends of the resonating arms across a center portion 440b of the diaphragm 440. The pedestals 444 and 445 are formed such that a width direction thereof in a direction orthogonal to the center portion 440b is decreased as they extend toward the center portion 440b. This allows the diaphragm 440 to easily bend as it extends toward the center portion 440b. Thus, displacement of the diaphragm 440 becomes large as it extends toward the center portion 440b when pressure is applied. Since the piezoelectric resonator element 431 easily bends in which the bases 436 and 437 are respectively bonded to the pedestals 444 and 445 at the both ends of the resonating arms across the center portion 440b, it is possible to realize a pressure sensor element capable of measuring pressure with high sensitivity.
These pressure sensors have a structure such that displacement of the diaphragm is transmitted to a pressure sensing element, such as a double-ended tuning fork element. Thus, higher sensitivity can be obtained as the displacement with respect to pressure to be measured is larger. In recent years, it has been proposed that the pressure sensor having enhanced accuracy is utilized for tire pressure monitoring systems (TPMS) for vehicles or height difference detecting devices for car navigation systems.
However, acceleration is generated at an instant at which a vehicle runs onto a step when the vehicle is moving, especially running at high speed on a highway. Then, in addition to a bend of the diaphragm of the pressure sensor due to pressure to be measured, a bend due to the acceleration is further added. This causes variations of an unnecessary bend due to the acceleration in resonance frequency of the pressure sensing element. As a result, errors are caused in a pressure detection value detected by a pressure sensor.